Automatic gauge control system for tandem rolling mills

ABSTRACT

Method and apparatus for varying the gain of an automatic gauge control loop for tandem rolling mills wherein final output gauge is controlled by varying the speed of the last stand in the tandem mill, while tension is controlled between the last two stands by varying the screwdown setting of the last stand. The gain of the control loop is varied as a function of transport time between the bite of the rolls of the last stand and a thickness gauge positioned beyond the last stand. A gauge deviation signal measured in volts/percent error is multiplied by a last stand speed signal to produce a first error signal and by the square of the last stand speed signal to produce a second error signal. These two error signals are summed. At low speeds, the first error signal is effective to control last stand speed with reduced loop gain. However, at higher speeds, the second error signal becomes effective and increases the gain of the loop. Additionally, since the multipliers used to produce the first and second error signals may be inaccurate at very low speeds, the gauge deviation signal is also summed with the aforesaid two error signals and used to control the system at very low speeds.

United States Patent [1 1 Peterson et al.

[ June 26, 1973 AUTOMATIC GAUGE CONTROL SYSTEM FOR TANDEM ROLLING MILLS [75] Inventors: Robert S. Peterson; John W. Cook,

' both of Williamsville, N.Y.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Feb. 29, 1972 [21] Appl. No.: 230,284

[52] U.S. Cl. 72/16 [51} Int. Cl. B2lb 37/02 [58] Field of Search 72/8, 10, 11, 16, 72/19 [56] References Cited UNITED STATES PATENTS 3,049,036 8/1962 Wallace et al. 72/9 3,440,846 4/1969 Scott 72/11 Primary Examiner-Milton S. Merr A ttorney- F. H. Henson, James J. Wood etal.

[ 5 7 ABSTRACT Method and apparatus for varying the gain of an automatic gauge control loop for tandem rolling mills wherein final output gauge is controlled by varying the speed of the last stand in the tandem mill, while tension is controlled between the last two stands by varying the screwdown setting of the last stand. The gain of the control loop is varied as a function of transport time between the bite of the rolls of the last stand and a thickness gauge positioned beyond the last stand. A gauge deviation signal measured in volts/percent error is multiplied by a last stand speed signal to produce a first error signal and by the square of the last stand speed signal to produce a second error signal. These two error signals are summed. At low speeds, the first error signal is effective to control last stand speed with reduced loop gain. However, at higher speeds, the second error signal becomes effective and increases the gain of the loop. Additionally, since the multipliers used to produce the first and second error signals may be inaccurate at very low speeds, the gauge deviation signal is also summed with the aforesaid two error signals and used to control the system at very low speeds.

9 Claims, 4 Drawing Figures ,34 ,36 TENSION SCREW- TENSON CONTROL DO'WN REFERENCE 30 1 SI 2' S\3 4 l u I I C C I IO l 1 I kl I4 M4 M5 x SR4 SR5 SPEED SPEED REGULATOR REGULATOR MASTER I SPEED REFERENCE I2 26 S5 -'I6 GUAGE CONTROL 2O REFERENCE PATENTEDJUNZSIQH v 3 740 983 SHE 1 BF 2 ,34 ,as TENSION ScREw- TENSION CONTROL DOIWN REFERENCE o SI S2 S3 A t\ p, 32 28 G5 L i3 \1 l4 SR4 SR5 SPEED SPEED REGULATOR REGULATOR MASTER SPEED I REFERNCE I2 26 s DIS AUTOMATIC GUAGE CONTROL 2o REFERENCE GAIN db Odb 5 5 LOW HIGH SPEED SPEED AUTOMATIC GAUGE CONTROL SYSTEM FOR TANDEM ROLLING MILLS BACKGROUND OF THE INVENTION Final output gauge of strip material passing through a tandem rolling mill can be controlled by varying the speed of the last stand in the mill. In a gauge control system of this sort, the tension in the strip between the last two stands will vary as last stand speed is varied. This change in tension is sensed and used to vary the screwdown setting of the last stand until the tension is brought back to the desired value. That is, the change in tension resulting from a change in last stand speed causes a change in the last stand roll gap to give the correct strip gauge called for by the change in last stand speed.

If the speed of the last stand is changed an amount equal to a given percentage of its operating speed, the delivery gauge will usually change by the same percentage. Increasing last stand speed makes the gauge become thinner and decreasing the last stand speed makes the gauge heavier. Therefore, the gauge error signal to the automatic gauge control system is calibrated in volts/percent and multiplied by delivery stand speed which will result in a correct change in delivery stand speed to bring the delivery gauge to the desired value.

In a gauge control loop of this type, the strip material, after passing through the bite of the rolls in the last stand, must travel to a thickness gauge positioned, for example, about feet beyond the bite of the rolls. Thus, a deviation in gauge from desired gauge is not detected until the strip material has traveled 5 feet to the thickness gauge which then develops an error signal used to take corrective action. This gives rise to what is known as transport time required for the strip to travel between the bite of the rolls and the thickness gauge. At low speeds and long transport times, the response time or gain of the control loop should be low. Otherwise, because of the long time delay between detection of a gauge error and correction, a high gain system would become unstable (i.e., oscillate). At high speeds, on the other hand, the response time or gain of the control loop can and should be increased to achieve better gauge control.

In prior art control systems of this type for tandem rolling mills, it has been the practice to use a fixed gain coefficient for the control loop, and this was a low gain coefficient necessary for the worst operating condition, namely low rolling speeds.

SUMMARY OF THE INVENTION In accordance with the present invention, a method and apparatus are provided for controlling gauge at the output of a tandem rolling mill by varying the speed of the last stand, and wherein the gain of an automatic gauge control loop for varying last stand speed is increased as the speed of the mill increases and decreases as the speed of the mill decreases.

Specifically, the invention involves (1) measuring the gauge of strip material issuing from the last stand of a tandem rolling mill at a point beyond the last stand and producing a signal proportional to the actual measured gauge, (2) comparing the actual gauge signal with a desired gauge signal as determined by an operator to derive a gauge deviation signal (measured in volts/percent error) for varying the speed of the last stand, (3)

multiplying the gauge deviation signal by the speed of the last stand to derive a first error signal, (4) multiplying the gauge deviation signal by the square of the speed of said last stand to derive a second error signal, and (5) summing said first and second signals with the gauge deviation signal and utilizing the sum to control the speed of the last stand.

Due to the inaccuracy of the multipliers used to produce the first and second error signals at low speeds, the original gauge deviation signal is summed with the error signals in controlling the speed of the last stand. At low speeds, the two error signals derived by multiplication are ineffective and the gauge deviation signal itself is used to control the mill. At higher, intermediate speeds, the first error signal is primarily effective in controlling the mill at relatively low gain. However, as the speed of the mill increases, the second error signal derived by multiplication with the square of last stand speed becomes effective to control the speed of the mill at high gain. In this manner, the gain of the control loop is optimized for all operating conditions.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic diagram of a tandem rolling mill operation incorporating the automatic gauge control system of the present invention in block form;

FIG. 2 is a Bode plot of rolling speed versus gain, showing the manner in which the gain increases as a function of speed in accordance with the system of the invention;

FIG. 3 is a detailed block diagram of the automatic gauge control system of the invention; and

FIG. 4 illustrates in greater detail the gauge control system of the invention.

With reference now to the drawings, and particularly to FIG. 1, a five-stand tandem rolling mill is shown including five stands S1, S2, S3, S4 and S5, only the rolls for stands S4 and S5 being shown in full lines since these are the only stands with which the automatic gauge control system of the invention in concerned. Strip material 10 to be rolled passes between the rolls of the successive stands 81-55 and is progressively reduced in gauge while the speed of the strip material increases at the output of each stand. The rolls for each of the stands are provided with drive motors, only the motors M4 and M5 being shown in FIG. 1. Motors M4 and M5 are controlled by speed regulators SR4 and SR5, respectively, which receive a master speed reference signal on lead 12 from a master mill speed control, not shown.

The gauge of the strip material 10 issuing from the last stand S5 is measured by an X-ray gauge 14 of the like which produces a signal on lead 16 proportional to actual gauge. The signal from X-ray gauge 14 is compared at summing point 18 with a gauge reference signal on lead 20 determined by the operator of the mill, or possibly by a computer; this gauge reference signal being proportional to the desired output gauge. If the desired gauge signal on lead 20 is not equal to the actual gauge signal on lead 16, an error signal calibrated in volts/percent is developed which is applied to an automatic gauge control circuit 22, hereinafter described in detail.

Also applied to the automatic gauge control circuit 22 is a signal derived from a tachometer or pulse generator 24. This signal is proportional to the circumferential speed of the last stand S and, hence, the speed of the strip material issuing from the mill. The output signal from the automatic gauge control circuit 22 is then summed with the master speed reference signal at point 26 and acts to modify the master speed reference signal as applied to the speed regulator SR5, depending upon whether a gauge deviation from desired gauge exists.

As was explained above, the system of the present invention controls final output gauge of the strip material issuing from stand S5 by varying the speed of the last stand S5. When the speed of stand S5 is thus varied, the tension in the strip between the last two stands will vary also. A variation in tension between the last two stands S4 and S5 changes the roll force on the last stand S5 which changes the mill stretch, which in turn changes the roll gap to the value required to roll the strip on gauge. This variation in tension is sensed by a tensiometer 28 in engagement with the strip material between stands S4 and S5 to produce a signal which is fed back to a summing point 30 where it is compared with a tension reference signal on lead 32. If the tension reference signal on lead 32 is not equal to that from tensiometer 28, an errorsignal is applied to tension control circuit 34 which, in turn, adjusts the screwdown mechanism 36 f0! stand S5 .until the actual tension matches the desired tension. In-other'words, the change intension occasioned by a change in stand 85 speed is used to vary the screwdown setting of the last stand until the tension is brought backto the desired value.,;

As is well known to those skilled in the art, a constant volume of strip per unit of time enters and leaves the The aforesaid-nonlinear Equation (2) relating strip thickness 7",, to the laststand speed S -can be linearized around an operating point where thelast stand speed 8,, equals S and delivery gauge or thickness T equals T This linear equation representsthe lineartransfer function showing the relationship of delivery gauge AT, to stand S and speed AS, for small perturbations of stand S, speed. In this respect: Let the transfer function Kg be:

Therefore:

( 8 o) T= o/ o) From perturbation theory:

and r r 5 o/ o) s Equation (6) above represents the linear transfer function relating delivery gauge T to stand S5 speed S,, which is the fixed plant of the control system. To maintain a constant automatic gauge control loop response with change in mill product requires that the gauge control system gain be multiplied by stand S5 speed S and divided by strip delivery thickness T If a volts/percent X-ray gauge error signal is used, the automatic gauge controller and gauge sensor combination gain is divided by delivery thickness. Additionally, multiplication of the automatic gauge controller gain by stand S5 speed S will compensate for changes in stand S5 speed.

The transport time, T, between the X-ray gauge 14 and stand S5 is the determining factor of how fast the gauge control loop can be at low speed. Of course, at lowthreading speeds, the 'strip being rolled for a given length of time is approximately one-twentieth the same length of strip being rolled at the maximum mill speed which, for example, may be 5000 feet per minute. As explained above, therefore, it is desirable that the automatic gauge control loop response become faster as the mill speed is increased to maintain strip gauge. This is possible since the transport delay between delivery gauge location and stand decreases directly with increase in mill speed. At top mill speed, the transport time T is no longer the limiting factor in the automatic gauge control .loop response. That is, at top speed, the

loop response of the last stand 85 is the main factor that gov'ernshowfa st the delivery: automatic gauge control loop can be. I g I I "As' wasexplained above, the present invention provid es'a system whereby the gain of the gauge control loop is varied as the speed of the last stand increases or decreases, to thereby optimize'g'ain for any operating speed. Th'eautoma'tic gauge control system 22 of FIG. 1 isshown in'idetail FIGS. 3 and 4. The actual and desiredgauge referencejsignals 16 and l8 are compared at summing point 18as' described above to developanierrorsignal which is app li'ed to an error compensation amplifier40. As shown by the transfer function thereonfthe"amplifier40 will produce an output linearlsignal which varies above or below a zero refer- .enc'ejdepe'ndinguponthe magnitude and polarity of the input error signal. This error signal is applied to a first inu'ltiplier 42where it is multiplied with thesignal SQ fromi Itac ho'm eter'f24 (FIG I) proportional to'the speed of standSS. The output signal from multiplier 42 isthen applied through aflrst potentiometer 44 to an automatic gain control correction amplifier 46.

The output of the multiplier 42 is applied to a second multiplier 48 where it is again multiplied with the stand S5 speed-signal 5,. Consequently, the output of multiplier 48 comprises the original gauge deviation signal at the: output of amplifier 40 multiplied by the square of standSS speed (i.e., 8.). This is applied through a second potentiometer 50 to the amplifier 46. Finally, the original gauge deviation signal at the output of amplifier 40 is applied directly'through potentiometer 52 to the amplifies.

, The details of the amplifiers 40 and 46 and the potentioineters 44, 50 and 52 are shown in FIG. 4. Thus, amplifier 40 comprises an operational amplifier 54 having two, feedback paths includingresistor 56 and a limiter 58 which limits the maximum output of the amplifier above and below the zero reference. One input to the operational amplifier 54 is connected through resistors 60 and 62 to the summing point 18; while the other input to the amplifier 54 is connected through resistor 64 to ground. The opposite ends of the resistor 60 are connected through capacitor 66 and resistor 68 to ground, as shown.

Each of the potentiometers 44, 50 and 52 is provided with a movable tap connected through resistors 70, 72 and 74, respectively, to a summing point 76. Point 76 is connected through resistors 80 and 82 to ground. Point 76 is also connected to one input of operational amplifier 78; while the other input to the operational amplifier 78 is connected through resistor 84 to ground. The amplifier 78 is provided with a feedback path including resistor 86 and capacitor 88. Also connected between the input and output of amplifier 78 is a variable limiter 92 which, in response to a function of stand S5 speed signal on lead 94, limits the maximum output of the circuit 46 as applied to the stand S5 speed controller. The limiter voltage reaches its maximum value at approximately 5 percent of stand S5 maximum speed and goes to zero linearly as stand SS speed goes to zero. The limiter prevents the delivery automatic gauge control system from trying to control delivery gauge at very low mill speeds (i.e., below 5 percent maximum speed). At this low speed, the gauge usually is too heavy for the system to correct and the automatic gauge control system might increase the tension be-.

tween stands S4 and S5to the point of strip breakage. This condition is prevented by reducing the controller amplifier 78 limits at very low operating speeds of stand The loop response of the automatic gauge control system at very low speeds (e.g., below 500 feet per minute) is determined by the setting of potentiometer 52. As was explained above, the potentiometer 52, which applies a signal at the output of circuit 40 to the input of circuit 46, is necessary since the static multipliers 42 and 48 are inaccurate at low input voltages which occur at very low speeds, such as threading speeds. Therefore, the potentiometer 52 bypasses the multipliers 42 and 48 and is used to adjust the loop response of the automatic gauge control system at thread speed (e.g., 300 feet per minute). If, however, the static multipliers 42 and 48 were perfect, this gain pot adjustment would .not be required.

At relatively low rolling speeds (e.g., 500 feet per minute), the automatic gauge control loop response is determined by the setting of potentiometer 44. As the speed varies at relatively low speeds, so will the voltage at the tap on potentiometer 44, thereby automatically varying gain upwardly or downwardly as speed increases or decreases, respectively. At this time, however, the output from multiplier 48 will be very small since it comprises the original deviation signal multiplied by the square of the speed. At high speeds, how- 7 ever, the signal at the output of multiplier 48 becomes large; and, hence, the setting of potentiometer 50 controls. However, the gain will again increase or decrease as speed increases and decreases. At all times, the signals on the taps of potentiometers 44, 50 and 52 are summed and applied to circuit 46.

The correction amplifier circuit 46 is a proportional plus integral controller which contains an integrator insuring zero steady-state strip gauge error and a lead time constant which compensates for the major time delay of the last stand speed loop which is approximately 0.3 second or less. At high mill speeds, the secondary time delay (approximately 0.1 second and smaller) of the speed loop has break frequencies of approximately 10 radians per second. In most cases, this will limit the response of the automatic gauge control loop to a crossover frequency of approximately 5 radians per second as is shown by curve 96 in FIG. 2. At low speeds of approximately 300 feet per minute, the transport time delay between delivery gauge 14 and stand S5 which is approximately 1 second limits the crossover frequency to about 0.5 radians per second as illustrated by curve 98 of FIG. 2.

The present invention thus provides a means, incorporating multiplication of a gauge deviation signal by a speed signal, for varying the gain of an automatic gauge control system wherein the gauge is controlled by varying the speed of the last stand in a tandem rolling mill. Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In the method for controlling final output gauge of strip material passing through a tandem rolling mill by varying the speed of the last stand in said tandem mill, the steps of:

measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and producing a signal proportional to the actual measured gauge,

comparing said actual gauge signal with a desired gauge signal to derive a gauge deviation signal for varying the speed of said last stand, modifying said gauge deviation signal as a function of the speed of said last stand to derive a first error Signal,

modifying said gauge deviation signal as a function of the speed of said last stand to derive a second error signal, and

combining said error signals and controlling the speed of said last stand as a function of the combined error signals.

2. The method of claim 1 including the step of combining the said first and second error signals and said gauge deviation signal and controlling the speed of said last stand with the combined first and second error signals and the gauge deviation signals.

3. The method of claim 1 wherein said gauge deviation signal is multiplied by the speed of said last stand to derive the first error signal and said gauge deviation signal is multiplied by the square of the speed of said last stand to derive the second error signal.-

4. The method of claim 3 including the step of varying the screwdown setting of said last stand as the speed of said last stand is varied.

5. Apparatus for controlling the final output gauge of strip material passing through a tandem rolling mill by varying the speed of the last stand in said tandem mill, comprising an automatic gauge control loop including means for measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and for producing a signal proportional to the actual measured gauge, means for comparing said actual gauge signal with a desired gauge signal to derive a gauge deviation signal for varying the speed of said last stand, a speed regulator for said last stand, and means for modifying said gauge deviation signal and for applying the modified gauge deviation signal to said speed regulator, the gauge deviation signal being modified as a function of the speed of said last stand to vary the gain of said automatic gauge control loop as the speed of said last stand varies.

6. The apparatus of claim wherein said means for modifying comprises means for multiplying said gauge deviation signal by the speed of said last stand to derive a first error signal, means for multiplying said gauge deviation signal by the square of the speed of said last stand to derive a second error signal, and means for combining said first and second error signals and for applying the combined error signals to said speed regulator.

7. The apparatus of claim 6 including means for combining the original gauge deviation signal as derived from said comparing means with said first and second error signals and for applying the combined first and second error signals and the gauge deviation signal to said speed regulator.

8. The apparatus of claim 6 including an operational amplifier for applying said gauge deviation signal to said multiplying means.

9. The apparatus of claim 8 including an operational amplifier for applying the combined error signals to said speed regulator, a feedback path for said lastnamed operational amplifier having a variable limiter therein, and means responsive to the speed of the said last stand so as to prevent automatic gauge control from causing strip breakage between last two stands of the mill for controlling said variable limiter. 

1. In the method for controlling final output gauge of strip material passing through a tandem rolling mill by varying the speed of the last stand in said tandem mill, the steps of: measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and producing a signal proportional to the actual measured gauge, comparing said actual gauge signal with a desired gauge signal to derive a gauge deviation signal for varying the speed of said last stand, modifying said gauge deviation signal as a function of the speed of said last stand to derive a first error signal, modifying said gauge deviation signal as a function of the speed of said last stand to derive a second error signal, and combining said error signals and controlling the speed of said last stand as a function of the combined error signals.
 2. The method of claim 1 including the step of combining the said first and second error signals and said gauge deviation signal and controlling the speed of said last stand with the combined first and second error signals and the gauge deviation signals.
 3. The method of claim 1 wherein said gauge deviation signal is multiplied by the speed of said last stand to derive the first error signal and said gauge deviation signal is multiplied by the square of the speed of said last stand to derive the second error signal.
 4. The method of claim 3 including the step of varying the screwdown setting of said last stand as the speed of said last stand is varied.
 5. Apparatus for controlling the final output gauge of strip material passing through a tandem rolling mill by varying the speed of the last stand in said tandem mill, comprising an automatic gauge control loop including means for measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and for producing a signal proportional to the actual measured gauge, means for comparing said actual gauge signal with a desired gauge signal to derive a gauge deviation signal for varying the speed of said last stand, a speed regulator for said last stand, and means for modifying said gauge deviation signal and for applying the modified gauge deviation signal to said speed regulator, the gauge deviation signal being modified as a function of the speed of said last stand to vary the gain of said automatic gauge control loop as the speed of said last stand varies.
 6. The apparatus of claim 5 wherein said means for modifying comprises means for multiplying said gauge deviation signal by the speed of said last stand to derive a first error signal, means for multiplying said gauge deviation signal by the square of the speed of said last stand to derive a second error signal, and means for combining said first and second error signals and for applying the combined error signals to said speed regulator.
 7. The apparatus of claim 6 including means for combining the original gauge deviation signal as derived from said comparing means with said first and second error signals and for applying the combined first and second error signals and the gauge deviation signal to said speed regulator.
 8. The apparatus of claim 6 including an operational amplifier for applying said gauge deviation signal to said multiplying means.
 9. The apparatus of claim 8 including an operational amplifier for applying the combined error signAls to said speed regulator, a feedback path for said last-named operational amplifier having a variable limiter therein, and means responsive to the speed of the said last stand so as to prevent automatic gauge control from causing strip breakage between last two stands of the mill for controlling said variable limiter. 